Synthesis of alpha-amino acids

ABSTRACT

A method for synthesizing alpha amino acids proceeding through novel intermediates of the formulas: 
     
         R.sub.1 R.sub.2 C(OSOCl)CN, R.sub.1 R.sub.2 C(Cl)CN and [R.sub.1 R.sub.2 
    
      C(CN)O] 2  SO 
     wherein R 1  and R 2  are each selected from hydrogen monovalent substituted and unsubstituted hydrocarbon radicals of 1 to 12 carbon atoms. The use of these intermediates allows the synthesis steps to be exothermic and results in an overall synthesis method which is faster than the synthesis methods of the prior art.

BACKGROUND OF THE INVENTION

This invention relates to a new method for synthesizing alpha aminoacids in which novel intermediates are utilized. More particularly, theprincipal distinguishing reaction is that of a cyanohydrin with thionylchloride. The invention was made in the course of or under U.S.Department of Energy Contract No. W-7405-ENG-48 with the University ofCalifornia.

This is a division of application Ser. No. 077,811, filed Sept. 21,1979, now abandoned.

Alpha amino acids have been synthesized for a number of years. A majoruse of alpha amino acids is in vitamin supplements. Another use is innuclear medicine wherein a synthesized alpha amino acid is labeled witha ¹¹ C atom in the carboxyl group.

In U.S. Pat. No. 2,520,312 a method is disclosed for synthesizing aminoacids. In particular, a cyanohydrin is converted to a correspondingamino nitrile by the action of ammonia at high temperature and pressure,an endothermic step. Of course, a synthesis of shorter duration andavoiding reacting under high pressure would be desirable.

It is an object of this invention to provide a new method forsynthesizing alpha amino acids having both higher yields and lowerpressure requirements than the prior art.

Another object of the invention is a synthesis which proceeds morerapidly than those of the prior art.

Yet another object is an alpha amino acid synthesis in which each stepis exothermic so that the overall process is relatively fast.

SUMMARY OF THE INVENTION

In the present invention, a novel process for synthesizing alpha aminoacids employs as a reactant, thionyl chloride (SOCl₂), and proceedsthrough novel intermediates. The process generally includes the steps ofreacting an aldehyde or ketone with cyanide to generate a cyanohydrin,reacting the cyanohydrin with thionyl chloride at room temperature togenerate a novel 2-chlorosulfinyl nitrile, reacting the novel2-chlorosulfinyl nitrile with liquid ammonia to generate an alpha aminonitrile and hydrolyzing the alpha amino nitrile to produce an aminoacid.

The novel intermediates are of the formula:

    R.sub.1 R.sub.2 C(OSOCl)CN, R.sub.1 R.sub.2 C(Cl)CN, and [R.sub.1 R.sub.2 C(CN)O].sub.2 SO,

wherein R₁ and R₂ are each selected from hydrogen and monovalentsubstituted and unsubstituted hydrocarbon radicals of 1 to 12 carbonatoms as defined in more detail hereinafter and preferably methyl,ethyl, propyl, butyl or isobutyl. The last-mentioned sulfiteintermediate above is a bis compound which can be symmetrical orasymmetrical.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The general reaction scheme, scheme A, for the alpha amino acidsynthesis of this invention is as follows: ##STR1##

Among the monovalent substituted and unsubstituted hydrocarbon radicals,which R₁ and R₂ can be, are alkyl radicals (e.g., methyl, ethyl, propyl,butyl, isobutyl, decyl); aryl radicals (e.g., phenyl, naphthyl,biphenyl); alkaryl radical (e.g., tolyl, xylyl, ethylphenyl); aralkylradicals (e.g., benzyl, phenylethyl, and alkenyl radicals (e.g., vinyl,allyl, methallyl), and p-hydroxybenzyl. The R₁ R₂ C(OSOCl)CN compoundsare novel.

It is preferred that hydrocarbon radicals contains from 1-5 carbonatoms. It is further preferred that one of R₁ and R₂ be hydrogen.

Alternatively, but less preferred, the conditions of the thionylchloride reaction can be modified so that the reaction proceeds throughanother intermediate which is also novel. This alternative reactionscheme, scheme B, is ##STR2##

In addition, yet another alternative exists and follows the thionylchloride step of scheme A. This alternative which may prove advantageousinvolves an additional intermediate step which yields a novelintermediate. This further step, scheme C, is ##STR3##

In more detail, the synthesis of the invention according to scheme Abegins with reacting an aldehyde or ketone, both commonly available,with a metal cyanide, such as potassium or sodium cyanide, to yield acyanohydrin. This reaction can be carried out at room temperature andpressure by suspending the metal cyanide in anhydrous ether, dissolvingthe aldehyde or ketone in glacial acetic acid, and then adding thedissolved aldehyde or ketone to the suspended metal cyanide dropwise.The reaction mixture is cooled with an ice bath. An almost quantitativeconversion to the cyanohydrin occurs. The carrying out of this reactionin the absence of water is believed to be novel.

No special precautions are needed to exclude traces of water during thecyanohydrin generation. Water appears to be involved in the reaction,but only trace amounts appear necessary to drive it forward. Potassiumcyanide typically contains some moisture leading to the reaction:

    CN.sup.- +H.sub.2 O=HCN+OH.sup.-

    OH.sup.- +CH.sub.3 COOH=CH.sub.3 COO.sup.- +H.sub.2 O

To separate the cyanohydrin product, filtration and distillation can beutilized. By-product acetate ion precipitates as metal acetate, i.e.,potassium acetate. The presence of increased amounts of water, althoughnot adversely affecting the yield, results in precipitated potassiumacetate which is heavy and pasty and, hence, makes mixing difficult. Alarge volume of ether facilitates mixing and isolation of the product.

The precipitated potassium acetate can be removed by filtration from thesolution. The filter cake should be washed several times with smallportions of anhydrous ether which is added to the solution. The ether inthe solution can be evaporated at room temperature and reduced pressureto leave the cyanohydrin product.

The next step in the synthesis yields a novel 2-chlorosulfinyl nitrile.Cyanohydrin is added slowly to a preferably stoichiometric equivalentamount of thionyl chloride over a period of time. The mixture is stirredand preferably kept at room temperature or below by means of a waterbath. Higher temperatures favor the product of scheme B. If astoichiometric excess of thionyl chloride is used, the mixture ispreferably anhydrous. The reaction is rapid and smooth and a 90% oftheory conversion of the cyanohydrin to the chlorosulfinyl nitrileproduct can be obtained. The product can be separated by means offractional distillation. The product is removed as one of the overheadproducts.

Also formed by the reaction of thionyl chloride and a cyanohydrin aretwo other novel compounds. The residue of the distillation consistsalmost entirely of the sulfite corresponding to the chlorosulfinylnitrile. This sulfite can also be prepared by treating thechlorosulfinyl nitrile with an excess of formamide, HCONH₂ (scheme C).The sulfite residue can be converted to the chlorosulfinyl nitrile byrefluxing with SOCl₂ for 5 to 10 minutes or can be reacted with liquidammonia at room temperature and normal pressure to bypass thechlorosulfinyl nitrile and yield the amino nitrile corresponding to thechlorosulfinyl nitrile, the next intermediate in the synthesis.

Another compound formed by the reaction of thionyl chloride with acyanohydrin is the 2-chloronitrile (scheme C) corresponding to the2-chlorosulfinyl nitrile. This chloronitrile can be formedquantitatively by refluxing the reaction mixture of the cyanohydrin andthionyl chloride for 4 to 5 hours. After removal of excess SOCl₂, theproduct oil (chloronitrile) can be distilled at atmospheric pressure.This chloronitrile can be reacted with liquid ammonia at roomtemperature and normal pressure to yield the corresponding alpha aminonitrile, the next intermediate in the synthesis.

As indicated in preceding paragraphs, the next step is to convert thechlorosulfinyl nitrile to the corresponding 2-amino nitrile. Thisreaction is highly exothermic and therefor the chlorosulfinyl nitrile ispreferably added dropwise to anhydrous ammonia cooled with a dryice-acetone bath. A vigorous reaction occurs. Upon completion, theexcess ammonia is preferably removed by evaporation by allowing themixture to warm to yield the amino nitrile.

This amino nitrile, in turn, is converted to the amino acidcorresponding to this amino nitrile by refluxing the amino nitrile withsodium hydroxide. Alternatively, a mineral acid such as hydrochloricacid can be used for this hydrolysis, but base is preferred for mostamino acids because there is no apparent tar formation and achromatographically pure sample is obtained.

Below is a table of some common amino acids which can be synthesizedwith the process of this invention.

    __________________________________________________________________________    TABLE OF ILLUSTRATIVE ALPHA AMINO ACIDS                                       Aldehyde  Chlorosulfinyl                                                                              (Common Name)                                         or Ketone nitrile       Amino Acid                                            __________________________________________________________________________                            (Valine)                                              (CH.sub.3).sub.2 CHCHO                                                                  (CH.sub.3).sub.2 CHCH(OSOCl)CN                                                              (CH.sub.3).sub.2 CHCH(NH.sub.2)COOH                                           (Alanine)                                             CH.sub.3 CHO                                                                            CH.sub.3 CH(OSOCl)CN                                                                        CH.sub.3 CH(NH.sub.2)COOH                                                     (Tyrosine)                                            C.sub.6 H.sub.4 OHCHO                                                                   C.sub.6 H.sub.4 OHCH(OSOCl)CN                                                               C.sub.6 H.sub.4 OHCH(NH.sub.2)COOH                                            (Alanine-α Amino                                                        Butyric Acid)                                         CH.sub.3 CH.sub.2 CHO                                                                   CH.sub.3 CH.sub.2 CH(OSOCl)CN                                                               CH.sub.3 CH.sub.2 CH.sub.2 (NH.sub.2)COOH                                     (Norvaline)                                           CH.sub.3 CH.sub.2 CH.sub.2 CHO                                                          CH.sub.3 CH.sub.2 CH.sub.2 CH(OSOCl)CN                                                      CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2 (NH.sub.2)COOH    __________________________________________________________________________

EXPERIMENTAL PREPARATION OF VALINE

Isobutyraldehyde was purified by distillation just before use. Apurified grade of thionyl chloride was further purified by distillationfrom about 10% of its weight of boiled linseed oil. The ammonia wasdried by distillation from a small quantity of clean sodium. All othermaterials were reagent grade. All boiling points are uncorrected. Theinfrared spectra were determined on a Perkin Elmer 1R421 (liquid filmbetween KBr plates).

2-Hydroxyisobutyronitrile ##STR4##

A well stirred suspension of 50 g of potassium cyanide in 800 ml ofanhydrous ether was cooled in an icewater bath and 36.5 g ofisobutyraldehyde in 45 ml of glacial acetic acid was added dropwisewithin 1 hour. A light voluminous precipitate of potassium acetate beganto form immediately. After stirring for another hour the acetate wasremoved by filtration and the filter cake washed several times withsmall portions of anhydrous ether. The ether was removed from thecombined filtrate and washings using a rotary evaporator at roomtemperature and reduced pressure. The remaining oil,2-hydroxyisobutyronitrile, weighed approximately 50 g. It distilledwithout decomposition at 66°-67° C. and 0.1 mm pressure.

Anal.% Calcd. for C₅ H₉ ON: C, 60.60; H, 9.09; N, 14.14. found: C,60.62; H, 9.04; N, 14.12.

2-Chlorosulfinylisobutyronitrile ##STR5##

The cyanohydrin product (50 g) from the above preparation was added to118 g of thionyl chloride over a period of 30 minutes while the mixturewas stirred and kept at room temperature by means of a water bath. Whenthe evolution of HCl had ceased, the excess thionyl chloride was removedunder reduced pressure and the residue fractionated to yield an almostcolorless oil, 2-chlorosulfinylisobutyronitrile, bp 40°-41° C. and 0.1mm pressure as one of the cuts.

Anal.% Calcd. for C₅ H₈ O₂ NSCl: C, 33.06; H, 4.40, S, 17.63; Cl, 19.53;N, 7.71. found: C, 33.36; H, 4.48, S, 17.56; Cl, 19.51; N, 7.70.

The yield for a number of runs varied between 80 and 86 grams. Theresidue which remained was distilled and consisted almost entirely ofthe sulfite corresponding to the chlorosulfinyl nitrile, bp 97°-98° C.at 0.1 mm pressure.

Anal.% Calcd. for C₁₀ H₁₆ O₃ N₂ S: C, 49.18; H, 6.56; N, 11.47; S,13.11. found: C, 49.52; H, 6.63; N, 11.61; S, 12.86.

2-Chloroisobutyronitrile ##STR6##

Alternatively, the chlorosulfinyl nitrile (50 g) as prepared above wasrefluxed with 60 g of thionyl chloride for five hours after which theexcess thionyl chloride was removed at atmospheric pressure. The residuewas distilled at 149°-150° C. at atmospheric pressure to yield acolorless oil, 2 chloroisobutyronitrile, weighing 30 g.

Anal.% Calcd. for C₅ H₈ NCl: C, 51.08; H, 6.81; N, 11.91; Cl, 30.18.found: C, 51.31; H, 6.85; N, 12.15; Cl, 29.86.

Isobutyronitrile sulfite ##STR7##

Also alternatively, the chlorosulfinyl nitrile (30 g) was added to 30 mlof formamide and the mixture shaken for several minutes until theresulting exothermic reaction was complete. The mixture was poured intowater (100 ml) and the oil extracted with ether. The ether solution waswashed twice with two 20 ml portions of water and dried over anhydroussodium sulfate. Upon removal of ether and distillation of the residuethere was obtained 18 g of a colorless oil, isobutyronitrile sulfite, bp97°-98° C. at 0.1 mm.

Anal.% Calcd. for C₁₀ H₁₆ O₃ N₂ S: C, 49.18; H, 6.56; N, 11.47; S,13.11. found: C, 49.09, H, 6.62; N, 11.59; S, 13.15.

Valine ##STR8##

To approximately 35 ml of anhydrous ammonia cooled with a dryice-acetone bath was added dropwise 18 g of the2-chlorosulfinylisobutyronitrile. A vigorous reaction occurs and whencomplete, the cooling bath was removed and the ammonia allowed toevaporate. To the resulting residue was added 75 ml of absolute ethylalcohol and the mixture heated to reflux. On cooling, 20 g of NaOH in100 ml of water was added and the temperature increased to above 90° C.allowing the alcohol to distill off. The mixture was refluxed for 24 hr.After cooling, 100 ml of 6 N HCl was added and the mixture taken todryness under reduced pressure. A few ml of water was added to theresidue and it was again taken to dryness. The residue was extractedseveral times with a total of 200 ml of hot absolute ethyl alcohol. Thealcoholic solution was concentrated to approximately 50 ml, filtered andtreated with 15 ml of pyridine. After standing in the refrigeratorovernight the crystals were collected, washed with alcohol and airdried. The yield for several runs was from 8 to 9 g of very pure almostcolorless valine. Paper chromatography showed the sample to behomogeneous having the same Rf value as a standard sample of valine(n-butanol; acetic acid; water; pyridine; 10, 2, 2, 1).

Anal.% Calcd. for C₅ H₁₁ O₂ N: C, 51.28; H, 9.4; N, 11.96. found: C,50.90; H, 8.96; N, 11.99.

I claim:
 1. An improved process for synthesizing an alpha-amino acidcomprising:reacting a metal cyanide with an aldehyde or a ketone in theabsence of water to form a corresponding cyanohydrin of the formula:

    R.sub.1 R.sub.2 C(OH)CN

wherein R₁ and R₂ represent hydrogen, alkyl radicals of 1 to 10 carbonatoms, alkenyl radicals of 2 to 4 carbon atoms, aryl radicals of 6 to 12carbon atoms, alkaryl radicals of 7-8 carbon atoms, aralkyl radicals of7-8 carbon atoms, and p-hydroxyphenyl radicals, reacting saidcyanohydrin with thionyl chloride to form a chlorosulfinyl nitrile ofthe formula:

    R.sub.1 R.sub.2 C(OSOCl)CN,

reacting said chlorosulfinyl nitrile with ammonia to form an aminonitrile of the formula:

    R.sub.1 R.sub.2 C(NH.sub.2)CH;

and, hydrolyzing said amino nitrile to the alpha-amino acid.
 2. Aprocess according to claim 1 wherein R₁ is hydrogen and R₂ is an alkylradical of 1 to 10 carbon atoms.
 3. A process according to claim 1wherein R₁ and R₂ are each an alkyl radical of 1-10 carbon atoms.
 4. Aprocess of claim 1 wherein R₁ and R₂ are each selected from hydrogen,methyl, ethyl, propyl, butyl, isobutyl, benzyl, and p-hydroxyphenyl. 5.A process according to claim 4 or 1 wherein one of R₁ and R₂ ishydrogen.